Not so long ago, Hillary Clinton supporters’ main concern seemed to be a fear that her coronation as the Democratic candidate for president would leave her unprepared for battle with the Republican nominee.

Now, by all metrics, the former secretary of state retains a historically strong lead in the race to secure her party’s nomination. She is well ahead of the other declared candidates in terms of poll numbers, money and endorsements. But a succession of setbacks and the possibility of another mainstream rival joining the race has, to some degree, checked the presumptive-ness of the presumptive candidate.

Despite the preponderance of evidence that suggests the nomination is Clinton’s to lose, doubts are cropping up.

“Hillary Clinton is always inevitable until the moment she isn’t,” said Republican strategist Rick Wilson. He called Clinton the “safe haven” for the Democratic party, but said speculation that other candidates were actively considering a campaign was a signal of her potential vulnerability.

“The argument for Hillary is this tautological ‘We have to support Hillary because it’s Hillary’,” Wilson said. “But it falls apart when it starts to get confronted with a more serious candidate or candidates.”

On Monday, as Clinton shook hands with hundreds of supporters who turned up at a ski resort in New Hampshire to hear her speak, Vermont senator Bernie Sanders packed a sports arena i n Los Angeles with more than 15,000 people .

Despite all this, many strategists, including Wilson, do not see Sanders as a viable threat. Biden, though, could make for a tight primary.

“If there is no Joe Biden, it is likely Hillary Clinton,” said Hank Sheinkopf, a New York Democratic strategist and former aide to President Bill Clinton.

And if there is a Biden candidacy?

“Anything can happen here, including her not being the nominee,” Sheinkopf said.

Biden is reported to be consulting supporters about a possible run, and has indicated that he will decide later this summer whether to throw his hat into the ring. It has been reported that he was urged to run by his son, Beau, who died in May after a lengthy battle with cancer.

Whether the 72-year-old vice-president will prove a serious threat to Clinton’s well-financed and well-organised campaign remains questionable, but his entry would certainly shake up the race.

“Vice-president Joe Biden appears to be much more likely to enter the contest than he was a while ago,” Sheinkopf said, “because his people see the obvious, which is Hillary Clinton may be very wounded in another couple of months and there may be an opportunity to enter the race.”

Sheinkopf said it was unreasonable to think that the average American voter was following every twist of the the long-rumbling saga over Clinton’s emails, but with new stories continuing to emerge, bad press may take its toll.

Earlier this week, Clinton campaign spokeswoman Jennifer Palmieri sent a message to supporters that emphasised that the candidate had been permitted to use a personal email account while serving as the Obama administration’s top diplomat. Clinton has turned over more than 30,000 emails to the State Department for review. On Monday, it was reported that two emails in a sample of 40 contained information classified as “top secret”.

On Friday, the Associated Press reported that the two emails considered “top secret” include a discussion of a news article detailing a US drone operation and a separate conversation that could point back to highly classified material in an improper manner – or merely reflect information collected independently.

Unless the email scandal escalates, the possibility of Clinton losing the nomination remains remote, said Simon Rosenberg, president of the left-leaning think tank NDN.